Cellulose esters are widely used in various applications such as filter tow, pharmaceutical, coatings, printing of several substrates, protecting hard surfaces, molding plastics, forming clear sheets and industrial coatings. In addition, cellulosic polymers which have affinity for water are of immense commercial interest. Water-swellable or water-absorbent polymers are used in absorbent pads, diapers, and various consumer goods. Water-soluble polymers are employed in cosmetics, foods, oil-field application such as oil recovery, drilling etc., and pharmaceutical coatings.
Cellulose acetates with a low degree of substitution (number of substituents per anhydroglucose ring) have high affinity for water. Several references disclose the preparation of water-soluble cellulose esters. For example, British Patent No. 356,012 discloses a process for preparing cellulose monoacetate (CMA) by sulfuric acid catalyzed hydrolysis of cellulose triacetate (CTA) in aqueous sulfuric acid. The product has a degree of substitution of 0.6-0.8 acetyl and is soluble in water. U.S. Pat. No. 2,327,770 discloses a process for preparing cellulose monoacetate by hydrolyzing cellulose diacetate in aqueous acetone or aqueous alcohol using sulfuric acid catalyst. Disadvantages of using the process disclosed in these patents include the use of a nonsolvent to isolate the product, long reaction times, and the necessity for continuous or sequential addition of water to maintain reaction rates which results in a dilute reaction mixture and difficulties in recovery of the by-product acetic acid.
U.S. Pat. No. 2,005,383 discloses a process for solvolyzing cellulose triacetate (CTA) using zinc iodide in ethanol. The product has a degree of substitution about 1.75. Disadvantages associated with this process are long reaction times (40 hours) and large amounts of zinc iodide (10 parts ZnI per part CTA) are consumed in the process.
U.S. Pat. No. 2,836,590 discloses high temperature (&gt;180.degree. C.) alcoholysis of cellulose acetate without the use of catalysts. At the temperatures disclosed, cleavage of the 1,4-glycosidic linkages of the cellulose ester backbone competes with the desired deacylation. U.S. Pat. No. 4,543,409 discloses a process for preparing cellulose monoacetate by acetylating cellulose in a solution of N,N-dimethylacetamide and lithium chloride. The cellulose monoacetate prepared by the process disclosed in U.S. Pat. No. 4,543,409, however, is not soluble in water.